December 19, 2017
As a metal alloy composed of nickel and titanium in approximately equal atomic percentages, Nitinol exhibits the unique properties of super-elasticity (SE) and shape memory effect (SME). These properties are due to a phase transformation (a change in crystal structure). Below the transformation temperature the microstructure is known as martensite, while above this temperature it is known as austenite.
Nitinol displays SE above its transformation temperature, due to martensite forming in areas that are stressed, even though the temperature is above where this normally occurs. When the stress is removed, this martensite returns to the undeformed austenitic state. While most metals can tolerate only a small fraction of a percent of strain without permanent deformation, Nitinol can take up to an eight percent strain and return to its original shape.
Similarly, SME enables Nitinol to revert to its original structure after deformation. When processed above its transformation temperature and given a specific shape, Nitinol’s shape memory property enables it to revert to its original shape when deformed below its transformation temperature. Simply heating the Nitinol causes the martensite to transform back to the undeformed austenite.
Healthcare and medical device applications are increasingly making use of Nitinol’s properties including ear implants, eyeglass frames and stents. A medical stent needs to be severely compressed so that it can be inserted into an artery, but when the compression is removed it springs out to support and hold open the artery.
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